1. Field of the Invention
The present invention relates to a bone cement formed of homopolymerizable plastics materials, particularly polymethyl methacrylate and its derivatives.
2. Description of the Prior Art
Since about 1960, it has been known to secure endoprostheses, particularly joint prostheses in bones, by means of homopolymerizable bone cements which are polymerized in situ. Polymethyl methacrylate has been widely used as the major constituent of such bone cement. These conventional polymethyl methacrylate cements generally comprise a methyl methacrylate homopolymer or copolymer in combination with a biocompatible reactive monomer such as methyl methacrylate. The systems may also comprise polymerization initiators and cross-linking agents. Such materials are well known in the art and need not be described in detail herein.
The cement is charged into the body aperture while it is still in its plastic polymerization phase. In this phase, it adapts ideally both to the prostheses parts and to the roughness of the bone. At the end of the polymerization phase, the bone cement sets and secure mechanical locking of the prosthesis body in the bone tissue occurs by means of the cement jacket. The bone cement also has a low modulus of elasticity which has the advantage that deformations and relative movement between the prosthesis and the bone tissue occurring when the prosthesis is used are reduced. Nevertheless, in the course of time, aseptic slackening of the prosthesis occurs, which is brought about by a number of factors. Insufficient mechanical strength of the bone cement is one factor, as is insufficient fatigue stability of the cement. Yet another factor is bone resorption caused by the body's reaction to chemical toxic foreign bodies. These foreign bodies are residual monomers and oligomers present in the polymethyl methacrylate cement system.
There have been attempts to increase the mechanical strength of the bond formed by a polymethyl methacrylate bone cement, for example, by inserting a woven basket at the prosthesis-to-bone interface, as described in the U.S. Pat. No. 4,064,567 to Burstein et al. The woven basket disclosed in this patent is formed from material which is compatible with body tissue and is described as interwoven metal wire in the form of an open screen structure. Among the metals disclosed as being suitable for the basket material are titanium, stainless steel, and chrome-cobalt. This patent discloses that plastics and carbon are also contemplated for use as the woven basket material.
Another technique for improving the mechanical strength of a polymerizable bone cement is to uniformly disperse short, high modulus graphite fibers into the cement, as described in U.S. Pat. No. 4,064,566 t Fletcher et al. While the procedure taught by this patent may improve bond strength, it gives rise to several disadvantages. For example, although this patent discloses the use of cements having from 2 to 12% by weight of graphite fibers, use of the preferred amount of fibers, i.e., about 10%, results in a curing time of about one hour. Such a long polymerization time involves unduly long operation times and exposure to infection, however, because an incision cannot be closed until after the bone cement has hardened. This problem is compounded in the case of a total prosthesis implant since it would be necessary to wait until the bone cement has hardened at the cup as well as at the shaft, thus prolonging an operation by as much as two hours merely waiting for the cement to harden. At lower graphite fiber contents, for example, at about 4%, the polymerization time is shortened considerably, but the peak polymerization temperature is increased above the critical temperature of 56.degree. C, known as the protein-denaturation temperature. Accordingly, the use of a bone cement containing 4% graphite fibers will result in heat damage to the environment tissue, accompanied by atrophy and resorption of the bone which leads to slackening of the prosthesis.
While the mechanical strength of a bone-to-prosthesis bond increases with the insertion of a woven basket or graphite fibers in the bone cement, the problems of bone resorption caused by the body's reaction to chemical toxic foreign bodies remains. To solve this particular problem the German Offenlegungsschrift No. 25 02 884 proposes to admix with the bone cement resorbable material such as collagen fibers or bands or cancellous parts. This material is slowly absorbed by the bone, and new bone tissue is built in its place. Other patents disclosing the addition of resorbable material in bone replacements, prosthesis anchoring materials, plastic bone compositions, dental implants or the like include U.S. Pat. Nos. 4,192,021, to Deibig et al, 3,789,029 to Hodosh and 4,051,598 to Sneer. In each of these patents, the disclosed system involves the continuous exchange of resorbable material with new bone tissue. However, the adjunction of said material, alone, will not result in the desired degree of mechanical strength.